linux-kernel - Re: [PATCH] KVM: X86: Expose KVM_HINTS_REALTIME in KVM_GET_SUPPORTED

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Date:   Fri, 23 Oct 2020 10:03:27 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: X86: Expose KVM_HINTS_REALTIME in KVM_GET_SUPPORTED_CPUID

On Fri, Oct 23, 2020 at 2:07 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 22/10/20 19:13, Jim Mattson wrote:
> > We don't actually use KVM_GET_SUPPORTED_CPUID at all today. If it's
> > commonly being misinterpreted as you say, perhaps we should add a
> > KVM_GET_TRUE_SUPPORTED_CPUID ioctl. Or, perhaps we can just fix this
> > in the documentation?
>
> Yes, I think we should fix the documentation and document the best
> practices around MSRs and CPUID bits.  Mostly documenting what QEMU
> does, perhaps without all the quirks it has to support old kernels that
> messed things up even more.
>
> Paolo

I'd really like to be able to call an ioctl that will help me
determine whether the host can support the guest CPUID information
that I already have (e.g. on the target of a live migration). At first
glance, KVM_GET_SUPPORTED_CPUID appeared to be that ioctl. Sadly, it
appears that no such ioctl exists.